fall over sb/sth 在行走时...脚碰到某人或某物而趔趄或跌倒
There is too much snow on the ground. If you ride much too fast,you will fall over. 地上的雪太多,如果你骑得太快的话,你会摔跤的。
fall down强调的是“滑倒、倒下”,后接宾语时应加上介词from。
The boy hit the tree to hard that he fell down. 男孩重重地撞在树上,结果自己倒下了。
fall off强调的是“跌落从……掉下来”,后直接接宾语。
The girl is falling off the bike. (= The girl is falling down from the bike.)女孩正从自行车上摔下来。